Apparatus for measuring displacement and method of use thereof

ABSTRACT

A sensor system for measuring displacement includes a primary coil wound around a longitudinal axis and at least one secondary coil wound around the longitudinal axis. Each secondary coil has a winding density distribution that varies between the ends thereof. The winding direction of each secondary coil varies between a clockwise winding direction between the ends thereof. A coupler is positioned adjacent the primary coil between the ends thereof. The coupler includes a resonating circuit configured to resonate at a resonating frequency. The coupler or the coils are configured to move relative to the other of the coupler or the coils. A control system excites the primary coil with a first step of a signal and receives from each secondary coil in response thereto a time varying signal. Each time varying signal includes a ringing component superimposed on a time varying component temporally adjacent the first step of the signal. The control system acquires plural samples of each time varying signal after the ringing component thereof dissipates and determines therefrom a position of the coupler along the longitudinal axis.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a CIP of U.S. patent application Ser. No. 09/057,811, filed Apr. 9, 1998 which is a CIP of U.S. patent application Ser. No. 09/044,300, filed Mar. 19, 1998, which is a CIP of PCT Application No. PCT/US96/15092, filed Sep. 19, 1996, which is based on U.S. Provisional Patent Application Ser. No. 60/003,969, filed Sep. 19, 1995.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a sensor system for monitoring fluid level and displacement and, more particularly, to a sensor system for monitoring the level of crude oil in storage containers and displacement of parts.

[0004] 2. Background Art

[0005] There are many types of sensors known in the art for monitoring fluid level and displacement of parts, and especially for measuring the level of crude oil in storage containers. Many of these sensors utilize a float designed to interact with the sensor. These sensors can be expensive in order to obtain the accuracy necessary and are often affected by the fluids and other contaminates associated with storage containers in which such fluids are stored. The following is a listing of known sensors for monitoring or measuring fluid level and disadvantages of such sensors.

[0006] Linearly variable differential transformers (LVDTs) require high precision manufacturing of the coils and a sensor length of more than two times the useful length of the sensor. They also have resolution that is limited by the resolution of their data acquisition system and by the electrical noise of the whole system.

[0007] Ultrasonic transducers are affected by changes in pressure, temperature and other variations in the composition of the media in which they operate due to their sensitivity to the density of the media. This limitation thereby increases the probability of errors.

[0008] Reed switch arrays, used in the oil industry, provide an incremental readout with limited resolution. However, they are sensitive to shock and vibrations and can be damaged by electrical storms. Furthermore, they are labor intensive to manufacture, which makes them expensive, and are unreliable due to the hundreds of switch contacts and internal connections. The accuracy of such arrays is typically +/−6.4 mm and clearance required between a float used with such reed switch arrays and the sensor elements must be between 0-3 mm. The arrays also require yearly cleaning and float replacement due to contaminant buildup.

[0009] Optical encoders are sensitive to contamination and are expensive. They also require high precision during manufacturing and implementation.

[0010] Magnetostrictive wave guide transducers are expensive and require high precision electronics. Also, the clearance between the float and the sensing element is limited.

[0011] Radar is expensive and has limited accuracy.

[0012] Capacitive probes are expensive and very sensitive to contamination. They also require high precision electronics and have a limited range.

[0013] Pressure transducers can be affected by contamination and have a resolution limited by the acquisition system employed.

[0014] It is an object of the present invention to provide a displacement measurement system that is inexpensive and easy to manufacture, has high reliability and accuracy, is easy implement, and has a low sensitivity to contamination, shock, electrical storms and the media in which it operates.

[0015] It is an object of the present invention to provide a displacement measurement system having an element that includes a resonator circuit.

[0016] It is an object of the present invention to provide a displacement measurement system having two or more elements, with each element having a resonator circuit responsive to a different frequency excitation signal.

[0017] It is an object of the present invention to provide a displacement measurement system having two or more elements, with each element having a resonator circuit responsive to a common excitation frequency, with each resonator circuit activatable independent of the resonator circuits of other elements.

[0018] It is an object of the present invention to provide a method for measuring displacement that avoids measurement errors caused by the displacement measurement system of the present invention or by interaction thereof with the environment in which it is installed.

SUMMARY OF THE INVENTION

[0019] In accordance with one aspect of the invention, a sensor system for measuring displacement includes a primary coil wound around a longitudinally extending axis. At least one secondary coil is wound around the longitudinal axis. Each secondary coil has a winding density distribution that varies between the ends thereof. Each secondary coil has a winding density distribution that varies between a clockwise winding direction and a counterclockwise winding direction between the ends thereof. A coupler is positioned adjacent the primary coil between the ends thereof. The coupler includes a resonating circuit configured to resonate at a resonating frequency. At least one of the coupler and the coils are configured to move relative to the other of the coupler and the coils. A control system excites the primary coil with a first step of a signal and receives from each secondary coil in response thereto a time varying signal. Each time varying signal includes a ringing component superimposed on a time varying component temporally adjacent the first step of the signal. The control system acquires one or more values of each time varying signal after the ringing component thereof dissipates and determines therefrom a position of the coupler along the longitudinal axis.

[0020] INSERT TAPE A . . . .

[0021] The control system can also determines from the one or more values of each time varying signal a first peak value after the ringing component dissipates and a second peak value after the first peak value. The control system can determine for each time varying signal a position signal value from the first peak value and the second peak value. From the position signal value for the time varying signal, the control system determines a position of the coupler along the longitudinal axis.

[0022] In response to the first step of the signal, the primary coil produces a first step electromagnetic field. In response to receiving the first step electromagnetic field, the resonating circuit produces a time varying electromagnetic field at the resonating frequency. In response to receiving the first step electromagnetic field and the time varying electromagnetic field, each secondary coil produces its corresponding time varying signal.

[0023] After acquiring the one or more values of the time varying signal, the control system can excite the primary coil with a second step of the signal having a transition opposite the first step of the signal. After the second step of the signal, the control system can determines from the one or more values of each time varying signal another first peak value after another ringing component thereof dissipates and can acquire another second peak value after the first peak value. For each time varying signal, the control system can determine another position signal value from the other first peak value and the other second peak value. From the other position signal value for each time varying signal, the control system can determine a position of the coupler along the longitudinal axis.

[0024] The control system can also determine from the one or more values for all or part of at least one half cycle of each time varying signal an average value or an integral value. For each time varying signal, the controller can determine a position signal value from the average value or the integral value, and from the position signal value for each time varying signal can determine a position of the coupler along the longitudinal axis.

[0025] After acquiring the one or more values of each time varying signal, the controller can excite the primary coil with a second step of the signal having a transition opposite the first step of the signal. Thereafter, the control system can acquire one or more other values of each time varying signal after another ringing component thereof dissipates. From the other one or more values, the controller can determine for all or part of at least one half cycle of each time varying signal another average value or another integral value. For each time varying signal, the controller can determine another position signal value between the other average values or the other integral values and from the other difference value for each time varying signal can determine a position of the coupler along the longitudinal axis.

[0026] INSERT TAPE B . . . .

[0027] In accordance with another aspect of the invention, a method of detecting the position of a magnetically susceptible element is provided. In the method, a primary coil wound around a longitudinal axis and at least one secondary coil wound around the longitudinal axis at a variable winding density distribution are provided. A coupler is positioned along the longitudinal axis adjacent the primary coil and each secondary coil. The coupler includes a resonating circuit configured to resonate at a resonating frequency. The primary coil is excited with a first step of a signal and a time varying signal is received from each secondary coil. Each time varying signal has a ringing component superimposed on a time varying component temporally adjacent the first step of the signal. One or more values of each time varying signal are acquired after the ringing component thereof dissipates. A position of the coupler along the longitudinal axis is determined from the one or more values of each time varying signal.

[0028] Preferably, for each time varying signal, a position signal value is determined from a first peak value and a second peak value and the position of the coupler along the longitudinal axis is determined from the position signal values.

[0029] In accordance with another aspect of the invention, a displacement measuring sensor system is provided. The sensor system includes a first member extending along a longitudinal axis. A primary coil and a secondary coil are wound around the longitudinal axis of the first member between the ends thereof. A control system is connected for exciting the primary coil with an excitation signal and for receiving a time varying signal from the secondary coil. The time varying signal includes a ringing component superimposed on a time varying component temporally adjacent a transition of the excitation signal. A second member is positioned on or adjacent the longitudinal axis. The second member includes a resonator which causes the secondary coil to generate the time varying component in response to the excitation of the primary coil with the excitation signal. The control system delays sampling of the time varying signal until the ringing component thereof dissipates.

[0030] Preferably, the first element or the second element are configured to move relative to the other of the first element and the second element.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a side elevational view of a fluid sensor system in accordance with the present invention positioned to detect the level of a fluid;

[0032]FIG. 2 is a sectional side elevational view of a portion of the sensor system in accordance with the present invention including a tube having coils thereabout and a float including a ferromagnetic core positioned in the float;

[0033]FIG. 3 is a sectional side elevational view of a portion of the sensor system in accordance with the present invention including the tube and coils illustrated in FIG. 2 and a float including an LC resonator therein;

[0034]FIG. 4 is a sectional side elevational view of the tube and coils of the sensor systems of FIGS. 2 and 3 including a toroidal shaped float having an LC circuit disposed therein;

[0035]FIG. 5 is a sectional side elevational view of a portion of the sensor system in accordance with the present invention including a tube having a primary coil, a monotonically varying secondary coil and a float, including a ferromagnetic core positioned in the float;

[0036]FIG. 6 is a block diagram of a control system for exciting a primary coil of the sensor system and for receiving signals from secondary coils of the sensor system;

[0037]FIG. 7 is a graph illustrating the winding density distributions including winding direction of a first and second secondary coil in accordance with an alternating step function and a third secondary coil having a linearly varying winding density distribution;

[0038]FIG. 8 is a graph illustrating the coil winding density distribution including winding direction of a first and second secondary coil in accordance with a sine and cosine function and a third secondary coil in accordance with a linearly varying function;

[0039]FIG. 9 is a sectional side elevational view of a portion of a fluid level sensor in accordance with the present invention including a primary coil, four secondary coils and a float that has a ferromagnetic core therein;

[0040]FIG. 10 is a sectional side elevational view of a portion of a fluid level sensor in accordance with the present invention including a primary coil, four secondary coils, a shield between the primary coil and secondary coils and a float that has a ferromagnetic core therein;

[0041]FIGS. 11a and 11 b are schematic representations of two manners of magnetically coupling the primary coil and the secondary coil;

[0042]FIG. 12 is a schematic representation of magnetic coupling between the primary coil, the secondary coils and a float having a resonating circuit made in accordance with the present invention;

[0043]FIG. 13a is a schematic representation of magnetic coupling between the primary coil, the secondary coils and two floats as shown in FIG. 12;

[0044]FIG. 13b is a timing diagram of the operation of the resonating circuits of the floats in FIG. 13a;

[0045]FIG. 14 is a schematic representation of magnetic coupling between the primary coil, the secondary coils and a float having dual resonating circuits made in accordance with the present invention each resonating circuit responsive to a different resonant frequency;

[0046]FIG. 15a is a schematic representation of magnetic coupling between the primary coil, the secondary coils and two floats as shown in FIG. 14;

[0047]FIG. 15b is a timing diagram of the operation of the resonating circuits of the floats in FIG. 15a;

[0048]FIGS. 16a-16 d are schematic views showing sensing systems for measuring displacement made in accordance with the present invention;

[0049]FIG. 17 is a sensing system for measuring displacement used in a valve;

[0050]FIG. 18 is a sensing system for measuring displacement made in accordance with the present invention for measuring a thickness of a part;

[0051]FIG. 19 is a sensing system for measuring displacement made in accordance with the present invention for measuring a gap between rollers;

[0052]FIG. 20 is a schematic view of a section of a coil having a ferromagnetic shield made in accordance with the present invention;

[0053]FIG. 21a is a schematic of a winding density distribution including winding direction of a variable secondary coil of a portion of a section made in accordance with the present invention;

[0054]FIG. 21b is a section of a portion of the sensor shown in FIG. 21a;

[0055]FIGS. 22a-22 d are diagrams of pulse signals applied to and signals responsive to the pulse signals received from a sensing system in accordance with the present invention;

[0056]FIGS. 23a-23 d are diagrams of step signals applied to and signals responsive to the step signals received from a sensing system in accordance with the present invention; and

[0057]FIGS. 24a-24 d are diagrams of periodic signals applied to and signals responsive to the periodic signals received from a sensing system in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] With reference to FIG. 1, a displacement measurement or inductive sensor system A of the present invention includes a tube 2 made of non-ferromagnetic material. A coupler or float 4, adapted to float on a fluid 6 disposed in a container 8 and adjacent tube 2, interacts with tube 2 in a manner that is to be described in greater detail hereinafter.

[0059] With reference to FIG. 2 and with continuing reference to FIG. 1, the tube 2 includes a plurality of coils layered coaxially and wound around or embedded into the tube 2 in a manner whereby each coil is electrically insulated from the other coils. The plurality of coils includes a primary coil 10, a first secondary coil 12 and a second secondary coil 14. The primary coil 10 is connected via connectors 15, or any other suitable arrangements for connecting the coils, to a control system 16 shown in FIG. 1 which provides a periodic driving signal to the primary coil 10. The secondary coils 12 and 14 are also connected via connectors 15 to the control system 16 which receives signals produced by the secondary coils 12 and 14 in response to changing magnetic flux passing therethrough. The control system 16 processes the received signals to a form to be analyzed by a monitoring computer.

[0060] A float 4′ traveling inside the tube 2 has a ferromagnetic material core 18. In response to the driving signal from control system 16, the primary coil 10 produces a time varying electromagnetic field which interacts with float 4′. The float 4′ modifies the electromagnetic field generated by the primary coil 10 to produce a locally intense electromagnetic field detectable by the secondary coils 12 and 14. The locally intense electromagnetic field detected by secondary coils 12 and 14 is converted by secondary coils 12 and 14 into electric signals which are detectable by signal-processing circuitry in the control system 16. The ferromagnetic core 18 intensifies the electromagnetic field by providing a low reluctance path for the electromagnetic field. Alternatively, the float 4′ can be formed from material having a reduced reluctance path relative to the tube 2, the aperture formed by the tube 2 or the fluid 6 in and surrounding the tube 2.

[0061] With reference to FIG. 3, in another embodiment, a float 4′ has an LC resonator C that includes a resonator coil 22 and a capacitor 24. The resonator coil 22 is preferably wound concentric with a longitudinal axis T_(A) of the tube 2. Electromagnetic fields produced by the primary coil 10 in response to excitation by control system 16 interact with the resonator coil 22 inducing oscillations in the LC resonator C. In response to the excitation frequency of the primary coil 10 being the same or near the resonance frequency of the LC resonator C, oscillations are produced in the LC resonator C. These oscillations produce a locally intense oscillating electromagnetic field in float 4′ that induces electric signals in the windings of the secondary coils 12 and 14 adjacent float 4′. The intensity of the oscillating electromagnetic field produced by the LC resonator C is approximately an order of magnitude greater than the intensity of the oscillating electromagnetic field produced by the interaction of float 4′ containing ferromagnetic core 18 with a comparable intensity electromagnetic field produced by the primary coil 10. The intensity of the oscillating electromagnetic field produced by float 4′ can be further increased if the resonator coil 22 of the LC resonator C is wound around a ferromagnetic core 26.

[0062] With reference to FIG. 4, a float 4′″ can also be formed as a toroid surrounding the tube 2, such as shown in FIG. 1. In this embodiment, the float includes a doughnut-shaped coil 30 surrounding the tube 2 and connected to a capacitor 32 thereby forming LC resonator D. The resonance frequency of the LC resonator D in float 4′″ is the same or near the excitation frequency of the primary coil 10. In response to the excitation of the primary coil 10 at or near the resonance frequency of the LC resonator D, oscillations are produced in the LC resonator D. These oscillations cause the LC resonator D to produce a locally intense oscillating electromagnetic field that induces electrical signals in the windings of first and second secondary coils 12 and 14 adjacent float 4′″. Alternatively, coil 30 can be wound around a ferromagnetic core 34.

[0063] The primary coil 10 is preferably wound at a constant winding density distribution, i.e., a fixed number of windings per unit length of tube 2, at least along the operative length of the tube 2, i.e., the length of the tube over which measurements are to occur. Alternatively, however, primary coil 10 can be wound with a variable winding density distribution providing the signals received by one or more secondary coils, described hereinafter in greater detail, are processed in a suitable manner to account for such variable winding density distribution in the primary coil 10. In one embodiment, the primary coil 10 is excited by a sinusoidal oscillating current supplied by control system 16 shown in FIG. 1. In response to such excitation, the primary coil 10 produces an oscillating electromagnetic field in and around the tube 2. Primary coil 10 can, alternatively, be excited by other excitation waveforms, such as, without limitation, a square wave or pulse waveform, described hereinafter in connection with FIGS. 22a-23 c.

[0064] The first secondary coil 12 is wound concentric with the primary coil 10 and at a density that, preferably, varies periodically along the longitudinal axis T_(A) of the tube 2. In the illustrated embodiment, the winding density distribution of the first secondary coil 12 is repeated a plurality of cycles between the ends of the primary coil 10. Alternatively, a single cycle of the winding density distribution of the secondary coil 12 can be utilized with relatively short tubes for accurate detection of the position of float 4′″ or with relatively long tubes for approximate detection of the position of float 4′″.

[0065] The winding density distribution of the first secondary coil 12 preferably varies as a sine function according to the equation:

Winding Density Distribution=a sine bx  EQ (1)

[0066] Alternatively, however, the winding density distribution can vary as another periodic function, such as, without limitation, a step or square function.

[0067] The winding direction of the first secondary coil 12 changes from clockwise winding to counterclockwise winding or vice versa, i.e., the coil is counter wound, at points along the length of tube 2 where the density of the coil equals zero. Preferably, the winding density distribution varies in accordance with equation 1 above and the coil arrangement is balanced, i.e., the number of clockwise windings equals the number of counterclockwise windings. In FIG. 4, the points where the coil density of the first secondary coil 12 are zero are identified as α, β, γ, δ and ε.

[0068] The signals induced in the windings of the first secondary coil 12 change periodically as a function of the excitation of the primary coil 10 and the axial position of float 4′″ in the tube 2. More specifically, the windings of the first secondary coil 12 that experience the greatest change in the magnetic flux are those windings closely adjacent float 4′″.

[0069] Alternatively, the first secondary coil 12 is wound in one direction only, e.g., clockwise, with periodic changing of the winding density. Winding first secondary coil 12 in this manner, however, results in first secondary coil 12 producing for a given intensity of electromagnetic field, additional unwanted signal components that must be accounted for when the signal produced by first secondary coil 12 is processed by the control system 16. Accordingly, a balanced coil arrangement is preferred.

[0070] The second secondary coil 14 is identical to the first secondary coil 12. The winding density distribution of the second secondary coil 14, however, is shifted with respect to the winding density distribution of the first secondary coil 12 by a part of a cycle of the winding density distribution or phase angle φ. Preferably, this phase angle φ is equal to one-quarter of the cycle of the winding density distribution for purposes of processing the signals produced by the first and second secondary coils 12 and 14. The points where the winding density distribution of the second secondary coil 14 are zero are identified in FIG. 4 as α′, β′, γ′, δ′ and ε′.

[0071] Referring back to FIG. 2, in operation, when primary coil 10 is driven by a sinusoidally varying signal, the sinusoidally varying signal in primary coil 10 produces a sinusoidally varying electromagnetic field in the ferromagnetic core 18 of float 4′. In response to the improved magnetic permeability of ferromagnetic core 18 in float 4′, a locally intense electromagnetic field is produced in and around tube 2 adjacent float 4′. This locally intense electromagnetic field produces in secondary coils 12 and 14 signals that have a phase and amplitude that can be translated into a sine function and a cosine function, respectively. The ability to translate the signals in secondary coils 12 and 14 into a sine and cosine function, respectively, is occasioned by the shift of one-quarter of the cycle of the winding density distribution between the first and second secondary coils 12 and 14. These sine and cosine or quadrature signals can be analyzed in a manner known in the art to determine the position of float 4′ in the tube 2 within one cycle of the winding density distribution of first and second secondary coils 12 and 14.

[0072] Any periodically varying signal in primary coil 10 can be utilized to excite signals in the first and second secondary coils 12 and 14. However, a sinusoidally varying signal is the simplest to translate to position information while allowing for compensation in variations of the excitation voltage coupling efficiency.

[0073] When wound in the above-described manner, first and second secondary coils 12 and 14 allow for the position of float 4′ to be determined within one cycle of the winding density distribution of first and second secondary coils 12 and 14. Moreover, if the position of float 4′ is continuously monitored by the control systems 16, the absolute position of float 4′ in tube 2 can be determined. However, if the power to the control system 16 is removed and thereafter reapplied, the absolute position of float 4′ in tube 2 cannot be determined utilizing first and second secondary coils 12 and 14 exclusively. Accordingly, it is preferable to provide a third position signal to the control system 16 having a resolution of at least one-quarter of the cycle length of the first and second secondary coils 12 and 14. This third signal is provided by a third secondary coil 40 with a winding density distribution that varies linearly according to the equation:

Coil Density−Ax|B  EQ(2):

[0074] i.e., as a function of the position of the coil along the length of tube 2.

[0075] With reference back to FIG. 4, the coil winding direction of the third secondary coil 40 changes from a clockwise winding direction to a counterclockwise winding direction where the coil density equals zero, designated as σ in FIG. 4. Preferably, the coil winding density equals zero intermediate the ends of the third secondary coil 40. By winding the third secondary coil 40 in this manner, the signal produced by the third secondary coil 40 varies monotonically along the length of the third secondary coil 40. Utilizing the third secondary coil 40, the approximate position of float 4′″ along the length of tube 2 can be determined independent of the first and second secondary coils 12 and 14. The first and second secondary coils 12 and 14 are then utilized to detect the position of the float 4′″ in tube 2 within one cycle of the winding density distribution of the first and second secondary coils 12 and 14.

[0076] With reference to FIG. 5, one primary coil 50 and one secondary coil 52 are wound around tube 2. The secondary coil 52 has a winding density distribution that varies monotonically and can be utilized to detect the position of the float 54. Generally, a secondary coil with a large winding density distribution produces a greater signal than a secondary coil with a small winding density distribution for a given intensity of magnetic flux passing therethrough. Thus, by detecting the amplitude of the signal produced by the one secondary coil 52, the approximate position of the float 54 can be determined in the absence of additional secondary coils.

[0077] With reference back to FIG. 4, to increase the resolution of the sensor to detect changes in the position of the float 4′″ relative to the tube 2, however, it is necessary to provide additional secondary coils, e.g., first and second secondary coils 12 and 14, having a plurality of cycles of winding density distribution that are shifted with respect to each other as illustrated in, for example, FIG. 4. To improve the resolution of the sensor to detect changes in the position of float 4′″, the length of each cycle of the winding density distribution is decreased. Similarly, increasing the length of each cycle of the winding density distribution decreases the resolution of the sensor. By providing secondary coils of various combinations of winding density distribution, cycle lengths, phase angle φ, and the like, a sensor of desired length and resolution can be obtained.

[0078] With reference back to any one of FIGS. 2-4, the winding density distribution of the first secondary coil 12 and the second secondary coil 14 can, alternatively, be modified so that a combination of the signals produced by the first and second secondary coils 12 and 14 varies monotonically along the length of the primary coil 10 thereby avoiding the need for additional secondary coils. One such combination of signals by first and second secondary coils 12 and 14 includes the sum of the squares of the signals generated by these coils.

[0079] The winding density distribution of the primary coil 10 can also be wound at other than a constant winding density with corresponding adjustment of the manner in which the signals produced by first and second secondary coils 12 and 14 are processed. Modifying the winding density distribution of the primary coil 10 or of the first and second secondary coils 12 and 14, however, may not provide sufficient resolution for tubes 2 having longer lengths. To increase the resolution in applications having long tubes 2, in addition to first and second secondary coils 12 and 14, one or more additional secondary coils of different winding density distribution and/or cycle lengths can be added to the tube 2. In this manner, large movements of a float can be measured with high accuracy, e.g., better than one percent of the length of the cycle of the one or more additional coils, without the need for high stability electronic circuits or high resolution data acquisition systems.

[0080] The order of winding the primary and secondary coils 10, 12, 14, etc., on the tube 2 is arbitrary and does not affect the operation or performance of the sensor system A. Moreover, any one of the primary and secondary coils 10, 12, 14, etc., can be used as a primary coil because inductive coupling between coils is bi-directional. Accordingly, the role of the primary coil 10 and one of the first and second secondary coils 12 and 14 can be reversed providing one of the coils produces a time varying electromagnetic field and the other coil(s) receive(s) the same.

[0081] With reference to FIG. 6, and with continuing reference to FIGS. 2-4, the control system 16 for exciting the primary winding 10 and receiving signals from the secondary coils includes a signal or sine wave generator 60 providing an output to an amplifier 62 and a trigger circuit 64. The amplifier 62 amplifies the output of the sine wave generator 60 and provides the amplified signal to the primary coil 10. In response to the output of the sine wave generator 60, trigger circuit 64 provides a trigger signal to a sample delay timer 66. The sample delay timer 66 delays the sampling of the output of the first, second and third secondary coils 12, 14 and 40 for a predetermined delay interval. After the predetermined delay interval, sample delay timer 66 provides a signal to sample duration timer 68. In response to the output signal of sample delay timer 66, sample duration timer 68 outputs a signal to a sample and hold 70 for a predetermined interval. During the interval the sample duration timer 68 outputs a signal, sample and hold 70 samples the output of a four channel amplifier 72, which receives the output of the first, second and third secondary coils 12, 14 and 40 of tube 2. After termination of the output signal by the sample duration timer 68, a computer 74 samples the outputs of the sample and hold 70 via an analog to digital converter 76. Alternatively, the fourth channel of four channel amplifier 72 is utilized to amplify the output of an optional fourth secondary coil 38 wound on tube 2. The features and use of the fourth secondary coil 38 will be described hereinafter in connection with FIG. 9.

[0082] With reference to FIGS. 2 and 6, a test sensor system A, constructed in accordance with the present invention, includes a plastic tube having a length of 1.5 meters and an internal diameter of 40 mm. The primary coil 10 was wound with a constant winding density around 1264 mm of the length of tube 2 in the manner illustrated in FIG. 2. The first and second secondary coils 12 and 14 were counter wound around the tube 2 in a manner wherein the cycle of the winding density distribution of each of the secondary coils is 158 mm and the winding direction changes every 79 mm. The axial length of the ferromagnetic core 18 is float 4′ is 67 mm. The first and second secondary coils 12 and 14 are wound in a manner whereby their respective coil winding density distribution are defined by alternating step functions shifted by a quarter of a cycle. The third secondary coil 40 is wound with a winding density that varies linearly in accordance with equation 2. The first, second and third secondary coils 12, 14 and 40 are wound with the same length as the primary coil, i.e., 1264 mm.

[0083] In operation, the float 4′ is positioned inside tube 2 and primary coil 10 is driven by signal generator 60 via amplifier 62. In response to excitation by primary coil 10, the ferromagnetic core 18 of float 4′ produces a locally intense electromagnetic field that is sensed by the windings of first, second and third secondary coils 12, 14 and 40, which are closely adjacent float 4′. In response to sensing the locally intense electromagnetic fields, the secondary coils produce signals that are detectable by the control system 16. The signals produced by the secondary coils are supplied to amplifier 72 which receives the supplied signals and amplifies the same. Amplifier 72 supplies the amplified signals to the sample and hold 70. The output of sample and hold 70 is provided to the analog to digital converter 76 which converts the analog output of the sample and hold 70 to a digital equivalent. The output of the analog to digital converter 76 is suppled to the computer 74. The signals output by the sample and hold 70 are individually analyzed by computer 74 to obtain the position of the float 4′ to within one cycle of the winding density distribution of the first and second secondary coils 12 and 14. The computer 74 also evaluates the analog to digital converted signal from the third secondary coil 40 to obtain the approximate position of the float 4′ along the length of the tube 2. Any non-linearities in the signal within the cycle caused by deviations of the winding density distribution from the preferred sine/cosine combination is compensated for numerically by a program in the computer 74. The above-described test sensor system has a resolution and repeatability of measurements within 1 mm.

[0084] With reference to FIG. 7, in another test sensor system A, the winding density distribution of the first and second secondary coils 12 and 14 is defined by alternating step functions shifted by one-quarter of a cycle of the winding density distribution. In this test sensor system, however, the winding density distribution of the first and second secondary coils 12 and 14 is repeated four cycles and each cycle has a length of approximately 90 mm. The plastic tube 2 in this prototype has an inside diameter of 20 mm. A third secondary coil 40 was wound with a winding density that varies linearly in accordance with equation 2. Thus, the coil density of the third secondary coil equals zero intermediate the ends of the third secondary coil 40. Like the prior test sensors system, the first, second and third secondary coils 12, 14 and 40 are connected to supply a signal to amplifier 72. The amplifier 72 amplifies the signals supplied by the secondary coils and supplies the amplified signals to sample and hold 70. At select intervals, the sample and hold 70 is caused to sample the outputs of amplifier 72 and supply the sampled outputs to analog to digital converter 76. At select intervals, computer 74 samples the output of analog to digital converter 76 to obtain the analog to digital converted outputs of sample and hold 70 for each of the secondary coils. If additional resolution and/or a longer sensor is needed, then additional secondary coils, e.g., fourth secondary coil 38 in FIG. 9, can be included on tube 2 and can have cycles of winding density distributions sufficient in length to provide a desired amount of position resolution.

[0085] With reference to FIG. 8, alternatively, the winding density distribution of the first and second secondary coils 12 and 14 is defined by a sine function and a cosine function, respectively, and the winding density distribution of the third secondary coil 40 varies linearly. The coil winding direction of the secondary coils changes each time the winding density distribution equals zero.

[0086] Signals from the first and second secondary coils 12 and 14 can be processed in two methods. In the one method, the change in voltage in each of the first and second secondary coils 12 and 14 during a fixed phase of the oscillation of the excitation voltage is determined and utilized to determine the position of float 4′ in tube 2. In this method, the phase shift φ between the first and second secondary coils 12 and 14, is preferably one-quarter of the cycle of the winding density distribution. This one-quarter of a cycle phase shift φ establishes a quadrature relationship between the first and second secondary coils 12 and 14 whereby the first and second quadrature signals are produced thereby. If the winding density distribution of the quadrature coils varies sinusoidally, then variations in the excitation amplitude and coupling efficiency can be determined numerically by computer 74 thereby reducing errors in the measured displacement of the float 4′ in the tube 2.

[0087] In the other signal processing method, the amplitude of oscillations in the first and second secondary coils 12 and 14 is measured. In this method, the shift between the first and second secondary coils 12 and 14 is one-eighth of a winding cycle. This is so because there are two amplitude cycles for every phase cycle. The circuitry of control system 16 for analyzing the signals in this latter method is similar, however, the circuitry is more sensitive to variations in the excitation amplitude and the coupling efficiency.

[0088] With reference to FIG. 9, a fourth secondary coil 38 is wound around tube 2. The winding density distribution of the fourth secondary coil 38 varies linearly and is shifted with respect to the winding density distribution of the third secondary coil 40. Preferably, the shift is equal to one-quarter of a cycle of the winding density distribution. In an embodiment of this type where the primary coil 10 was wound with a constant winding density distribution, the first and second secondary coils 12 and 14 were wound twenty cycles of the winding density distribution thereof, and the third and fourth secondary coils 40 and 38 were wound with one cycle of the winding density distribution thereof, a resolution of {fraction (1/4000)} was obtained.

[0089] With reference to FIG. 10, the primary coil 10 is wound with a constant winding density distribution, the first and second secondary coils 12 and 14 are wound with a sinusoidally varying winding density distribution and the first and second secondary coils 12 and 14 are shifted by part of a cycle of the winding density distribution, preferably one-quarter of a cycle. The first and second secondary coils 12 and 14 are wound with N cycles of winding density distributions over the length of the tube 2. Moreover, a third secondary coil 80 and a fourth secondary coil 82 are wound relative to each other in a manner similar to first and second secondary coils 12 and 14, i.e., the third and fourth secondary coils 40 and 42 are shifted one-quarter cycle with respect to each other. However, the third and fourth secondary coils 80 and 82 are wound with N+1 cycles of winding density distributions over the length of tube 2.

[0090] To determine the position of float 4′ in this embodiment, the position of float 4′ within a cycle of the winding density distribution of the first and second secondary coils 12 and 14 is determined and the position of float 4′ within a cycle of the winding density distribution of the third and fourth secondary coils 80 and 82 is determined. Because there is no overlap in the winding density distributions between the first pair of secondary coils 12 and 14 and the second pair of secondary coils 80 and 82, the position of the float can be determined by comparing the relative position information of the first pair of secondary coils 12 and 14 with the relative position information of the second pair of secondary coils 80 and 82. Stated differently, the combination of the quadrature signals of the first pair of secondary coils and the second pair of secondary coils is unique for every position of the float 4′ over the length of the tube 2. Accordingly, the position of float 4′ in the tube 2 can be determined from the quadrature signals to within one-half of one percent of the length of the shorter of the cycle windings, i.e., N+1 cycle windings. Error averaging can be utilized to further improve the accuracy of the sensor system. An advantage of this latter embodiment is that the measurement of the position of the float 4′ is independent of the total length of one or more of the secondary coils, e.g., first and second secondary coils 12 and 14. This is particularly advantageous in applications where ferromagnetic material settles around one end of the tube 2 whereby the detection capability of a linearly varying coil would be affected by the magnetic particles. Alternatively, the first and second secondary coils 12 and 14 are wound with N cycles of winding density distributions and the third and fourth secondary coils 80 and 82 are wound with M winding density distributions, wherein N and M are mutually prime numbers that do not have a denominator in common other than the number one.

[0091] Preferably, the secondary coils have balanced windings, i.e., the number of windings in a positive direction equals the number of windings in a negative direction. Moreover, shielding 90, such as aluminum foil or wound coils, is alternatively provided between the primary coil and the secondary coils and/or between secondary coils. This shielding improves the signal output by the secondary coils by reducing the level of noise on the electromagnetic fields received by the secondary coils. When formed from wound coils, the shielding 90 can be counter wound between the ends of the tube 2. While described in conjunction with measuring fluid levels, the present invention also finds applications in replacement of LVDTs. Moreover, the choice of a doughnut-shaped float of FIG. 4 versus the piston-shaped float of FIG. 2 is application dependent. Specifically, for measuring the level of dirty fluids, a doughnut-shaped float is preferred because the gap between the float and the tube is more readily cleanable than the gap between the piston-shaped float and the inside of the tube.

[0092] With reference back to FIG. 1, since an LC resonator can be used in float 4, multiple floats with LC resonators tuned to different frequencies can be used to detect, for example, levels of water and oil in the same tank. Thus, one float 4 can be used to measure the level of oil on water while another float 5 shown in phantom in FIG. 1 can be used to measure the level of the water. The excitation frequency of primary coil 10 in FIG. 4 would be changed between readings in order to stimulate the proper float.

[0093] With reference to FIG. 11a, the primary coil 10 is magnetically linked to one or more of the secondary coils via the ferromagnetic core. With reference to FIG. 11b, the primary coil is magnetically linked to the secondary coil via the float containing the LC resonator. The diagrams in FIGS. 11a and 11 b illustrate two ways of magnetically coupling the primary coil and the secondary coils.

[0094] With reference to FIG. 12 and with reference back to FIG. 6, in another embodiment, the float 4 can include an LC resonator 100. The LC resonator 100 includes a resonator coil 102 connected in parallel to a series connected capacitor 104 and switch 106, such as an FET. The resonator coil 102 and the capacitor 104 form a resonating circuit R which is active when the switch 106 is closed and which is inactive when the switch 106 is open. A switch control circuit 108 includes a signal conditioning circuit 110 connected to the resonator coil 102. The signal conditioning circuit 110 amplifies signals generated by the resonator coil 102 and supplies the amplified signals to a CP input of a counter 112. The signal conditioning circuit 110 also generates a reset signal which is coupled to an MR input of the counter 112. The reset signal causes registers within the counter 112 to reset.

[0095] In operation, the sine wave generator 60 of the control system 16 excites the primary coil 10 of the tube 2 with a time varying signal at a first frequency, preferably 1.2 kHz. In response, the primary coil 10 produces a time varying electromagnetic field which causes the resonator coil 102 to generate a signal at the first frequency. The first frequency preferably corresponds to the resonating frequency of the resonating circuit R. The signal conditioning circuit 110 amplifies the signal generated by the resonator coil 102 and provides the amplified signal to the CP input of the counter 112. When the resonator coil 102 initially generates the signal at the first frequency, the signal conditioning circuit 110 terminates the reset signal to the MR input of the counter 112 thereby enabling the counter 112 to count each cycle of the amplified signal received at the CP input.

[0096] The counter 112 includes a plurality of outputs each of which changes state in response to the CP input of the counter 112 receiving a predetermined number of cycles of the amplified signal from the signal conditioning circuit 110. Specifically, the counter 112 has a first output 114 that changes state when the CP input receives 128 cycles of the amplified signal. The counter 112 also has second and third outputs 116, 118 which change state in response to the CP input receiving 256 and 512 cycles of the amplified signal, respectively. The counter 112 can include other outputs corresponding to the CP input receiving a predetermined number of counts associated therewith.

[0097] In the embodiment shown in FIG. 12, the second output 116, is connected to a switch control lead 120 which is connected to control the on/off operation of the switch 106. In response to the CP input receiving 256 cycles of the amplified signal from the signal conditioning circuit 110, the second output 116 of the counter 112 changes state thereby causing switch 106 to close, or conduct, and activate the resonating circuit R.

[0098] Cycles 0-255 of the amplified signal correspond to a first interval during which the control system 16 samples a compensation signal from each secondary coil 12, 14, etc. By each secondary coil 12, 14, etc., is meant that the sensor can have one or more of the previously described secondary coils, or others. This compensation signal corresponds to the response of each secondary coil 12, 14, etc., to excitation by the time varying electromagnetic fields produced by the primary coil when the resonating circuit R is inactive. Cycles 256-511 of the amplified signal correspond to a second interval during which the switch 106 is conducting and the resonating circuit R is active. The active resonating circuit R produces at each secondary coil 12, 14, etc., a locally intense time varying electromagnetic field which is detectable by the signal processing circuitry 70, 72, 74, 76 of the control system 16. During the second interval, the control system 16 samples a composite signal from each secondary coil 12, 14, etc. The control system 16 subtracts from the composite signal for each secondary coil 12, 14, etc., the compensation signal thereof to obtain an elemental signal corresponding to the actual response of each secondary coil 12, 14, etc., to the active resonating circuit R.

[0099] When the compensation signal and the composite signal have been obtained for each of the one or more secondary coils 12, 14, etc., the sine wave generator 60 terminates exciting the primary coil 10. In response, the signal conditioning circuit 110 terminates supplying amplified signals to the CP input of the counter 112 and provides the reset signal to the MR input of the counter 112. The reset signal causes internal registers of the counter 112 to reset.

[0100] With reference to FIGS. 13a and 13 b and with ongoing reference to FIG. 12, since the LC resonator 100 can be used in float 4, multiple floats with switch control circuits 108 tuned to count different numbers of cycles of the excitation frequency can be utilized to detect, for example, levels of water and oil in the same tank. For example, the float 4 described in connection with FIG. 12 can be utilized to detect the level of oil on water and a float 5, similar to float 4, can be utilized to detect the level of the oil. The elements of the float 5 are similar to the elements of the float 4, where like elements are identified by primed numbers.

[0101] In operation, the sine wave generator 60 initially excites the primary coil 10 at the first frequency. In response, the counters 112 and 112′ of the respective floats 4 and 5 begin counting cycles of the amplified signals provided at the first frequency to the CP inputs thereof. Cycles 0-255 of the first frequency correspond to a first interval 122. During the first interval 122, the second and third outputs 116, 118 of counters 112, 112′ connected to the switch control leads 120, 120′, respectively, do not change state thereby maintaining switches 106, 106′ open. During the first interval 122, the control system 16 samples the compensation signal from each secondary coil 12, 14, etc.

[0102] When the sine wave generator 60 has excited the primary coil 10 with 256 cycles at the first frequency, the second output 116 of the counter 112 changes state thereby causing the switch 106 to close and activate the resonating circuit R. Cycles 256-511 of the first frequency correspond to a second interval 124 during which the switch 106 is conducting and the resonating circuit R is active. During the second interval 124, the control system 16 samples a first composite signal from each secondary coil 12, 14, etc. When the sine wave generator 60 excites the primary coil 10 with 512 cycles at the first frequency, the second output 116 of the counter 112 changes state causing switch 106 to open. When switch 106 is open, the resonating circuit R is inactive. Simultaneously, the third output 118′ of the counter 112′ changes state thereby causing the switch 106′ to conduct and activate resonating circuit R′ for a third interval 126. During the third interval 126, the control system 16 samples a second composite signal from each secondary coil 12, 14, etc.

[0103] When the control system 16 has sampled the second composite signal from each secondary coil 12, 14, etc., the sine wave generator 60 terminates exciting the primary coil 10. In response, the signal conditioning circuits 110, 110′ of the floats 4 and 5, respectively, terminate supplying the amplified signals and, after a predetermined delay, generate reset signals to the MR inputs of the counters 112, 112′.

[0104] To detect the response of the secondary coils 12, 14, etc., to the resonating circuit R, the control system 16 subtracts the compensation signal for each secondary coil 12, 14, etc., from the first composite signal thereof to obtain a first elemental signal corresponding to the actual response of each secondary coil 12, 14, etc., to the active resonating circuit R. Similarly, the control system 16 subtracts the compensation signal for each secondary coil 12, 14, etc., from the second composite signal thereof to obtain a second elemental signal corresponding to the actual response of each secondary coil 12, 14, etc., to the active resonating circuit R′. Hence, by subtracting the compensation signal for each secondary coil 12, 14, etc., from the composite signal for each secondary coil 12, 14, etc., the elemental signal response of each secondary coil 12, 14, etc., to the resonating circuits R, R′ can be determined.

[0105] While described in connection with floats 4 and 5, additional floats can be provided to detect levels of additional different fluids in the tank.

[0106] With reference to FIG. 14, another embodiment of float 4 includes an LC resonator 130′ having a resonator coil 132, capacitors 133, 134 and a switch 136. A switch control circuit 138 includes a signal conditioning circuit 140 having an output connected to a CP input of a counter 142. A reset control 144 has an output connected to an MR input of the counter 142 for providing a reset signal thereto.

[0107] The resonator coil 132 has a tap 146. In a preferred embodiment, the resonator coil 132 includes 1800 turns and the tap 146 separates the resonator coil 132 into a first part TI having 1100 turns and a second part T2 having 700 turns. All 1800 turns of the resonator coil 132 are connected across the capacitor 133, the reset control 144 and a power supply 148, such as a rechargeable battery and/or a storage capacitor. The second part T2 of the resonator coil 132 is connected to the signal conditioning circuit 140, the counter 142 and the series connected capacitor 134 and switch 136. The second part T2 of the resonator coil 132 and the capacitor 134 form a resonating circuit S that is active when the switch 136 is closed and is inactive when the switch 136 is open.

[0108] In operation, the sine wave generator 60 excites the primary coil 10 with a signal having a first frequency, preferably, 3.125 kHz. The resonator coil 132 and the capacitor 133 form a resonator circuit having a resonant frequency corresponding to the first frequency. In response to excitation at the first frequency, the resonator coil 132 and capacitor 133 resonate and generate a signal to the reset control 144 and the power supply 148 at the first frequency. In response to the signal generated by the resonator coil 132 and the capacitor 133 at the first frequency, the reset control 144 generates a reset signal to the MR input of the counter 142. Simultaneously, components (not shown) of the power supply 148 are charged. The sine wave generator 60 excites the primary coil 10 with the signal at the first frequency for a first interval sufficient to cause the reset control 144 to reset the counter 142 and to charge the components of the power supply 148.

[0109] After the first interval, the sine wave generator 60 excites the primary coil 10 with a signal having a second frequency, preferably, 1.2 kHz. In response to excitation of the primary coil 10 at the second frequency, the second part T2 of the resonator coil 132 generates a signal to the signal conditioning circuit 140 and the counter 142 at the second frequency. The second frequency preferably corresponds to the resonating frequency of the resonating circuit S. In response to receiving the signal generated by the second part T2 of the resonator coil 132, the signal conditioning circuit 140 amplifies the signal and provides the amplified signal to a CP input of the counter 142. The counter 142 counts each cycle of the amplified signal received at the CP input. Cycles 0-255 of the second frequency correspond to a second interval wherein a first output 152 of the counter 142 is in a state whereby the switch 136 connected to the first output 152 by a switch control lead 156 remains open and the resonating circuit S is inactive. During cycle 256 of the second frequency, the first output 152 changes state causing the switch 136 to conduct thereby activating the resonating circuit S. During the second interval, the control system 16 samples from each secondary coil 12, 14, etc., a compensation signal. Cycles 256-511 of the second frequency correspond to a third interval wherein the resonating circuit S is active. During the third interval, the control system 16 samples from each secondary coil 12, 14, etc., a composite signal. To detect the response of each secondary coil 12, 14, etc., to the resonating circuit S, the control system 16 subtracts the compensation signal for each secondary coil 12, 14, etc., from the composite signal thereof to obtain an elemental signal corresponding to the actual response of each secondary coil to the resonating circuit S.

[0110] With reference to FIGS. 15a and 15 b and with ongoing reference to FIG. 14, the float 4 having the LC resonator 130 and the float 5 having an LC resonator 130′ are adapted to measure the respective level of oil on water and the level of water in the same tank. The LC resonator 130′ is similar to the LC resonator 130 where like elements are identified by like numbers. However, the second output 154′ of the counter 142′ is connected to the switch control lead 156′ of the switch 136′.

[0111] In operation, the sine wave generator 60 excites the primary coil 10 with a signal at the first frequency for a first interval 158. The first frequency causes the reset controls 144, 144′ to reset their respective counters 142, 142′ and causes components of the power supplies 148, 148′ to store electrical charge therein. After the first interval 158, the sine wave generator 60 excites the primary coil 10 with a signal at the second frequency. During a second interval 160 corresponding to cycles 0-255 of the second frequency, the switches 136, 136′ are open and the resonating circuits S, S′ are inactive. During the second interval, the control system 16 samples from each secondary coil 12, 14, etc., a compensation signal. During a third interval 162 corresponding to cycles 256-511 of the second frequency, the resonating circuit S′ is inactive and the resonating circuit S of the float 4 is active thereby producing a locally intense electromagnetic field. During the third interval 162, the control system 16 samples from each secondary coil 12, 14, etc., a first composite signal. During a fourth interval 164, which begins at cycle 512 of the second frequency, the resonating circuit S of the float 4 is inactive and the resonating circuit S′ of the float 5 is active. During the fourth interval 164, the control system 16 samples from each secondary coil 12, 14, etc., a second composite signal. To detect the response of each secondary coil 12, 14, etc., to the resonating circuit S, the control system 16 subtracts the compensation signal for each secondary coil 12, 14, etc., from the first composite signal thereof to obtain a first elemental signal corresponding to the actual response of each secondary coil 12, 14, etc., to the resonating circuit S. Similarly, to detect the response of each secondary coil 12, 14, etc., to the resonating circuit S′, the control system 16 subtracts the compensation signal for each secondary coil 12, 14, etc., from the second composite signals thereof to obtain a second elemental signal corresponding to the actual response of each secondary coil 12, 14, etc., to the resonating circuit S′.

[0112] With ongoing reference to FIGS. 12-15 b, it can be seen that the excitation frequency of the primary coil 10 causes each switch 106, 106′, 136, 136′ to be open during a first interval and to be closed during a second interval. The switches 106, 106′ have at least a portion of their respective first intervals occurring during a common interval of time. Similarly, switches 136, 136′ have at least a portion of their respective first intervals occurring during a common interval of time. Importantly, where two or more floats, e.g., 4 and 5, having resonating circuits R, R′ are utilized, the second interval of each float is adapted to occur at a different interval of time. Moreover, the embodiments shown in FIGS. 14 and 15a are also responsive to a frequency other than the resonant frequency of the resonating circuits S, S′ to charge the power supplies 148, 148′ and generate reset signals which reset the counters 142, 142′.

[0113] The embodiments described in connection with FIGS. 12-15 b can be applied to fluid level sensors having one or more secondary coils. Moreover, the activation of the resonating circuits R, R′, S, S′, as well as the interval utilized to obtain the compensation signal, can occur in any order. Moreover, each interval can be separated in time from other intervals.

[0114] With reference back to FIG. 4, in order to obtain the compensation signal for each of the secondary coils 12, 14, etc., when utilizing the LC resonator D, the float 4′″ is separated from the tube 2. The control system 16 then samples a compensation signal response of each secondary coil 12, 14, etc., to stimulation by time varying electromagnetic fields produced by the primary coil 10. The compensation signal for each secondary coil 12, 14, etc., is stored in a memory of the control system 16 for subsequent subtraction from the composite signal for each secondary coil 12, 14, etc., when the float 4′″ is adjacent the tube 2. Alternatively, the control system 16 excites the primary coil 10 at one or more non-resonant frequencies of the LC resonator D. The response of each secondary coil 12, 14, etc., is determined for each of the one or more non-resonant frequencies and suitable numerical adjustments are made thereto in order to determine an estimated compensation signal that corresponds to the compensation signal measured when the float 4′″ is separated from the tube 2.

[0115] With ongoing reference to FIG. 4, the third secondary coil 40 has one winding cycle between the ends of the tube 2 and has a winding density distribution that varies linearly between the ends of the tube 2. The third secondary coil 40 is wound whereby a float positioned adjacent one end thereof produces a positive peak, and the float adjacent the other end thereof produces a negative peak. If, however, the winding density distribution of the third secondary coil 40 is shifted by one-half cycle, the winding density distribution at opposite ends of the third secondary coil 40 are the same, i.e., zero. Hence, the third secondary coil 40 will generate the same signal when the float 4′″ is positioned at either end thereof. To avoid generating the same signal at opposite ends of the third secondary coil 40, the ends thereof are wound in a non-periodic manner. Specifically, the third secondary coil 40 is wound so that the coil density distribution at opposite ends thereof does not return to zero. Hence, the third secondary coil 40 can accurately detect the position of the float 4′″ at one end of the tube 2.

[0116] The tube 2 and the coils 10, 12, 14, etc., wound thereabout, can be rigid or flexible. To secure the primary coil 10 to the tube 2, a length of tape having an adhesive on one side thereof is wound around the tube 2 with the adhesive side exposed. The primary coil 10 is then wound onto the adhesive side of the tape in a desired manner. Similarly, another length of tape having adhesive on one side thereof is wound around the primary coil 10 with its adhesive side exposed. The first secondary coil 12 is secured to the exposed adhesive side in a desired winding pattern. Subsequent coils are wound around the tube 2 in a similar manner. It is to be appreciated that tape having adhesive on both sides can also be utilized. Moreover, when all of the coils have been wound around the tube 2 an electrical insulating sheath can be wrapped around the coils.

[0117] The sensor system provides a system with improved accuracy and fewer moving parts to increase reliability over the prior art, as well as providing a sensor that is not sensitive to float clearance, needs cleaning about once every three years, has a continuous readout, low cost of manufacture and a low sensitivity to shock and storms.

[0118] The present invention is also a displacement measurement system A for non-fluid applications. Specifically, the previously described floats can be fixed to a mechanical member, such as a movable arm or a fixed surface, and the respectively described tube/coil arrangements can be attached to a movable arm or a fixed surface.

[0119]FIGS. 16a-16 d show four arrangements of displacement measurement system A having a first member 200 including a primary coil 10 and one or more secondary coils 12 or other coils 14, 40, etc., and a second member 210 comprised of any one of the previously described couplers or floats 4, 4′, 4″, 4′″, etc. The second member 210, the first member 200 and the previously described control system 16 form a sensing system for measuring displacement.

[0120] Specifically, FIG. 16a shows a system 220 for measuring displacement where the first member 200 is adapted to move along an axis 222 and the second member 210 is ring shaped defining an opening and fixed to a surface 224. The first member 200 is adapted to move relative to the second member 210.

[0121]FIG. 16b shows a system 230 for measuring displacement similar to the sensing system 220 for measuring displacement, except the first member 200 is fixed to a surface and the second member 210 is adapted to move along an axis 234. The second member 210 is adapted to move relative to the first member 200.

[0122]FIG. 16c shows a system 240 for measuring displacement similar to the sensing system 220 for measuring displacement, except the second member 210 is secured to a rod 242 coupled to a surface 243. The second member 210, which can be plug shaped as shown in FIG. 3, is received within the first member 200, which is tubular shaped, such as shown in FIG. 3. In this arrangement, the first member 200 is adapted to move along an axis 244 relative to the second member 210.

[0123]FIG. 16d shows a system 250 for measuring displacement similar to the sensing system 230 for measuring displacement, except the second member 210, which can be plug shaped as shown in FIG. 3, is attached to a rod 252 and is adapted to move along an axis 254. The first member 200 is attached to a surface 256 and is tubular shaped as shown in FIG. 3 so that the second member 210 is received within the first member 200. In this arrangement, the second member 210 is adapted to move along the axis 254 relative to the second member 210. It should be noted that the axes 222, 234, 244 and 254 can be straight lines or curved and it is also possible that both the first member 200 and the second member 210 can move relative to each other.

[0124] The system for measuring displacement can be used in many applications, such as: automotive applications for position feedback, active suspension, throttle control or active power steering; robot automation feedback, such as arms and jaws; aircraft flap extenders, thrust reversers, wheel extenders or control surface management; hydraulic cylinder extension or retraction; pneumatic cylinder extension or retraction; large machines, such as wood lathes, tool position measurement and feedback; process equipment positioning and conveyor movement, such as for example: position measurement of hot steel ingots; food processing equipment, such as squeezers and crushers; earth moving machinery; fruit picking machines; control of automated paint sprayers and welders; use in areas of severe environments involving chemicals, vapors, heat, dust, etc. where optical systems cannot be maintained; use in spacecraft for position feedback; use for monitoring of structural components of a space station; use on satellites where long term reliability and low power consumption are required; feedback on solar panel positioning device; geomechanics, earthquake monitoring and damage protection; artificial limb position feedback; gaging; valve position feedback; part thickness measurements; roller gap measurements in steel mills, paper mills, etc.; and medical instrumentation.

[0125] More specifically, FIG. 17 shows a valve positioning arrangement utilizing the system 250 for measuring displacement that includes a flow pipe 260 in fluid communication with a gate valve 262 adapted to move along an axis 264 to adjust a fluid flow opening 266 moved by the rod 252 attached to a gate 268 of the gate valve 262, whereby the position of the second member 210 relative to the first member 200 determines the size of the flow opening 266.

[0126]FIG. 18 shows an arrangement for measuring a thickness of a part 270 relative to a reference point 271 utilizing the system 250 for measuring displacement including a part 272 having a part surface 274. An end 276 of the rod 252 is adapted to contact the part surface 274 causing the second member 210 to be positioned within the first member 200.

[0127]FIG. 19 shows an arrangement 280 for measuring a gap between rollers used, for example, in the metal industry or in paper mills, rubber, plastic sheets and film plants and includes a first roller 282 and a second roller 284. A gap 286 is defined between the rollers 282 and 284. System 250 for measuring displacement is coupled to the rollers 282 and 284 so that the rod 252 is coupled to the roller 282 and the second member 210 is coupled to the roller 284. The gap 286 corresponds to the position of the second member 210 within the first member 200. In the above arrangements, the first member 200 is coupled to an appropriate signal generator and signal processing circuit as previously discussed. Also, the signal processing circuit can be coupled to a control circuit and used for feedback control, for example, with the gate valve 262 (FIG. 17) and the rollers 282 and 284 (FIG. 19). Although, not shown, it is possible to utilize two second members 210 in combination with one first member 200 by using the arrangement such as disclosed in FIG. 15a and previously described herein. This arrangement can be used to measure the displacement of two parts relative to the first member 200.

[0128]FIG. 20 shows a resonator coil second member or a float 300 similar to the resonator coil shown in FIG. 4 further having a ferromagnetic shield 302 surrounding at least a portion of the coil 300. The resonator coil 300 which also includes a capacitor 304 can be positioned on either side of the shield 302. The shield 302 prevents extraneous effects to the sensing system disclosed due to close proximity of ferromagnetic objects to the resonator coil 300. The other second members 210 described herein can also have a ferromagnetic shield, if necessary.

[0129]FIGS. 21a and 21 b show a further embodiment of the first member 200 of the present invention that uses a primary coil 10 having constant windings per unit length, a compensating secondary coil 306 having constant windings per unit length and a variable secondary coil 308 with a winding density that varies linearly in accordance with equation 2. Thus, the coil density of the variable secondary coil 308 equals zero intermediate the ends of the variable secondary coil 308. The compensating secondary coil 306 assists in compensating for changes in the signal application of the variable secondary coil 308 due to effects caused by extraneous ferromagnetic objects. In operation, the first member 200 and the second member 210 are calibrated with a first time varying signal passing through the primary coil 10. A signal processing circuit (not shown) measures the generated signals through the secondary coils 306 and 308. The signal measured in secondary coil 306 is constant with respect to the position of the second member 210 and the signal measured in the secondary coil 308 varies with respect to the position of the second member 210 along the length of the first member 200. The secondary coil 306 can be calibrated to be a constant one volt and the secondary coil 308 can be calibrated to be between −1 volt and +1 volt depending on the position of the second member 210 relative to the first member 200. It has been found that should a ferromagnetic object be placed in close proximity to the first member 200, both of the secondary coils 306 and 308 will be affected equally. Therefore, after calibration and installation of a system utilizing the first member 200 and the second member 210, the system can become self-compensating should a ferromagnetic member affect the signal of the secondary coils 306 and 308. As an example, should the signal processor detect that the signal of the compensating secondary coil 306 reads 0.5 volts and its calibrated signal is 1 volt, then the voltage signal of the variable secondary coil 308 should be multiplied by a factor of 2 (1 divided by 0.5) to yield a compensated variable secondary signal. This assumes that the signal passing through the primary coil is constant. In other words, it has been found that the following relationship applies:

(calibrated voltage of the variable secondary coil 308) (calibrated voltage of the compensating secondary coil 306)=(measured voltage of the variable secondary coil 308) (measured voltage of the compensating secondary coil 306)

[0130] The compensating secondary coil 306 can be used with other secondary coils previously described with or without the variable secondary coil 308.

[0131] With reference to FIGS. 22i a-2 d and with reference back to FIGS. 3-4 and 16 a-16 d, to avoid measurement errors caused by the displacement measurement system A or interaction thereof with the measuring environment, the control system 16 can apply to primary coil 10 a pulse signal 310 having a first, leading step/edge 312 and a second, trailing step/edge 314. In response to primary coil 10 receiving pulse signal 310 in the absence of an LC resonator C or D adjacent primary coil 10, each secondary coil 12, 14 and/or 40 only produces an electrical oscillation or ringing 316. This ringing 316 is caused by the capacitive and inductive characteristics of primary coil 10 and each secondary coil 12, 14 and/or 40 in response to the excitation of primary coil 10 by leading step/edge 312 and trailing step/edge 314 of pulse signal 310.

[0132] If, however, LC resonator C or D is included in float 4″, float 4′″ or second member 210 positioned adjacent primary coil 10, in response to exciting primary coil 10 with pulse signal 310, each secondary coil 12, 14 and/or 40 produces a time varying signal 318 that includes a time varying component 320 and a ringing component 322, corresponding to ringing 316, superimposed on time varying component 320. Time varying component 320 of time varying signal 318 is due to the energy coupled into LC resonator C or D by the excitation of primary coil 10 with pulse signal 310. More specifically, in response to receiving the leading step/edge 312 of pulse signal 310, primary coil 10 produces a first step electromagnetic field, having a waveform similar to leading step/edge 312 of pulse signal 310, which is received by LC resonator C or D. This first step electromagnetic field couples energy into LC resonator C or D which begins oscillating at its resonant frequency thereby producing a time varying electromagnetic field which is coupled to each secondary coil 12, 14 and/or 40. The dissipation of ringing component 322 is due to the dissipation of energy input into primary coil 10 and coupled directly into each secondary coil 12, 14 and/or 40 by the excitation of primary coil 10 with leading edge/step 312 of pulse signal 310.

[0133] Next, in response to receiving trailing step/edge 314 of pulse signal 310, primary coil 10 produces a second step electromagnetic field, having a waveform similar to trailing step/edge 314 of pulse signal 310, which is received by LC resonator C or D. This second step electromagnetic field also couples energy into LC resonator C or D. Since leading step/edge 312 and trailing step/edge 314 of pulse signal 310 are temporally closely adjacent, the second step electromagnetic field received by LC resonator C or D reinforces the oscillation thereof.

[0134] In response to receiving the first step electromagnetic field, the second step electromagnetic field and the time varying electromagnetic field, each secondary coil 12, 14 and/or 40 produces time varying signal 318. The time varying component 320 of time varying signal 318 corresponds to the oscillation of LC resonator C or D and is related to the position of float 4″, float 4′″ or second member 210 along the longitudinal axis T_(A) of primary coil 10. In contrast, ringing component 322 of time varying signal 318 is related to energy input into primary coil 10 and the energy coupled directly into each secondary coil 12, 14 and/or 40 in response to primary coil 10 receiving pulse signal 310 and, more particularly, the leading step/edge 312 and the trailing step/edge 314 of pulse signal 310. Hence, ringing component 322 is not related to the position of float 4″, float 4′″ or second member 210 along the longitudinal axis T_(A) of primary coil 10.

[0135] As shown in FIG. 22c, ringing component 322 of time varying signal 318 commences and dissipates temporally adjacent pulse signal 310. In contrast, time varying component 320 of time varying signal 318 commences temporally adjacent pulse signal 310 and dissipates after one or more oscillation cycles of LC resonator C or D. The difference in duration of ringing component 322 and time varying component 320 is due to differences in the respective energy storage capacity and damping characteristic of primary coil 10, each secondary coil 12, 14 and/or 40 and LC resonator C or D. Specifically, since each primary coil 10 and secondary coil 12, 14 and/or 40 has less capacity to store energy and more damping characteristic than LC resonator C or D, any energy stored in primary coil 10 or stored in each secondary coil 12, 14 and/or 40 dissipates more rapidly than energy coupled into LC resonator C or D. In addition, the resonant frequency of time varying component 320 is substantially different than the resonant frequency of the capacitive and inductive characteristics of primary coil 10 and each secondary coil 12, 14 and/or 40. Hence, little or no energy is coupled into the capacitive and inductive characteristics of primary coil 10 and each secondary coil 12, 14 and/or 40 by time varying component 320 and, therefore, little or no additional ringing is superimposed on time varying component 320 by the capacitive and inductive characteristics of each secondary coil 12, 14 and/or 40.

[0136] To avoid measuring time varying signal 318 in the presence of ringing component 322, control system 16 delays sampling time varying signal 318 until ringing component 322 dissipates. Preferably, control system 16 delays sampling time varying signal 318 until time varying signal 318 crosses a reference voltage 324, e.g., 0 volts, after ringing component 322 dissipates. Thereafter, control system 16 samples for each secondary coil 12, 14 and/or 40 values of time varying signal 318 over time. For example, control system 16 can sample one or more peak values of time varying signal 318, e.g., a first peak value 326 of time varying signal 318 and/or a second, preferably, adjacent peak value 328 of time varying signal 318 for each secondary coil 12, 14 and/or 40. Alternatively, control system 16 acquires for each secondary coil 12, 14 and/or 40 a plurality of values of time varying signal 318 during all or part of one or more half cycles thereof and determines therefrom an average value or an integral value for each half cycle or part thereof. Since no additional energy is coupled into LC resonator C or D during sampling by control system 16, the amplitude of time varying signal 318 gradually decreases due to the dissipation of energy coupled into LC resonator C or D.

[0137] Next, control system 16 calculates for each secondary coil 12, 14 and/or 40 a position signal value which is equal to (i) one of the sampled peak values, e.g., peak values 326 or 328; (ii) a sum or difference between two or more adjacent or non-adjacent peak values, e.g., first peak value 326 and second peak value 328 of time varying signal 318; (iii) an average or integral value of all or part of one half cycle of time varying signal 318; or (iv) a sum or difference of the average or integral value of all or part of each of two or more half cycles of time varying signal 318. This position signal value is related to the position of float 4″, float 4′″ or second member 210 along the longitudinal axis T_(A) of primary coil 10. More specifically, since each secondary coil 12, 14 or 40 has a periodically varying winding density distribution, the number of turns thereof that are exposed to the time varying electromagnetic field produced by LC resonator C or D affects the amplitude of time varying signal 318 produced thereby. Thus, if float 4″, float 4′″ or second member 210 is adjacent a minimum winding density distribution of one of the secondary coils 12, 14 or 40, the time varying component 320 therefrom will have a minimum amplitude. In contrast, if float 4″, float 4′″ or second member 210 is adjacent a maximum winding density distribution of one of the secondary coils 12, 14 or 40, the time varying component 320 therefrom will have a maximum amplitude. Moreover, the time varying component 320 will vary between minimum amplitude and maximum amplitude for positions of float 4″, float 4′″ or second member 210 adjacent primary coil 10 between the minimum winding density distribution and the maximum winding density distribution of each secondary coil 12, 14 or 40. Thus, for each secondary coil 12, 14 or 40, the calculated position signal value corresponds to the position of float 4″, float 4′″ or second member 210 along the longitudinal axis T_(A) of primary coil 10 within one cycle of the winding density distribution of each secondary coil 12, 14 or 40.

[0138] It is contemplated that other techniques can be utilized to calculate a position signal value for each secondary coil 12, 14 and/or 40. Hence, the foregoing and after described techniques for calculating a position signal value are not to be construed as limiting the invention.

[0139] Utilizing two or more secondary coils 12, 14 and/or 40 having different winding density distributions and/or shifted winding density distributions enables control system 16 to accurately detect the position of float 4″, float 4′″ or second member 210 along the length of primary coil 10. For example, the position signal value obtained from secondary coil 40, having one winding density distribution between the ends of tube 2, enables the control system 16 to determine the approximate position of float 4″, float 4′″ or second member 210 along the longitudinal axis T_(A). In addition, the difference values obtained from secondary coils 12 and 14 wound as shown in FIGS. 3 and 4, enable the control system 16 to determine the position of float 4″, float 4′″ or second member 210 along the longitudinal axis T_(A) within one cycle of the winding density distribution of secondary coils 12 and 14.

[0140] As shown in FIGS. 22a-22 c, control system 16 preferably sequentially applies pulse signals 310 to primary coil 10. Each pulse signal 310 after the first pulse signal 310 is applied after control system 16 samples values of time varying signal 318 produced in response to an immediate preceding pulse signal 310. Sequentially applying pulse signals 310 to primary coil 10 enables control system 16 to continuously monitor the position of float 4″, float 4′″ or second member 210 along the length of tube 2.

[0141] Each pulse signal 310 is shown as a square pulse for illustration purposes since square pulses are the easiest to generate. However, the shape of pulse 310 is not to be construed as limiting the invention. In this respect, pulse signal 310 can have any shape that includes a leading edge and trailing edge, with the square pulse of pulse signal 310 being shown in FIG. 22a as an example of an envelope of all possible pulse shapes. In practice, a gaussian or bell-shaped pulse is preferred since it generates less ringing in each primary coil 10 and/or in secondary coil 12, 14 and/or 40. Alternatively, a half sine wave 329, shown in phantom in FIG. 22d, can be applied to primary coil 10. In response to receiving a leading step/slope 360 of half sine wave 329, primary coil 10 produces a first step electromagnetic field, having a waveform similar to leading step/slope 360 of half sine wave 329, which is received by LC resonator C or D. This first step electromagnetic field couples energy into LC resonator C or D and each secondary coil 12, 14 and/or 40 in the same manner as the first step electromagnetic field produced in response to the leading step/edge 312 of pulse signal 310. Similarly, in response to receiving the trailing step/slope 362 of half sine wave 329, primary coil 10 produces a second step electromagnetic field, having a waveform similar to trailing step/slope 362 of half sine wave 329, which is received by LC resonator C or D. This second step electromagnetic field couples energy into LC resonator C or D and each secondary coil 12, 14 and/or 40 in the same manner as the second step electromagnetic field produced in response to the trailing step/edge 314 of pulse signal 310. The form of step leading step/slope 360 and the form of trailing step/slope 362 of half sine wave 329 reduce the amplitude of the time varying component 320 of time varying signal 318 and reduce the amplitude and spectral frequency of ringing component 322 of time varying signal 318. This reduction in amplitude and spectral frequency of ringing component 322 is due to the lower energy and spectral content of half sine wave 329 versus the energy and spectral content of the square wave of pulse signal 310.

[0142] With reference to FIGS. 23a-23 d, and with continuing reference to FIGS. 3-4 and 16 a-16 d, the control system 16 can also apply to primary coil 10 a step signal 330 having a leading step/edge 332 and a trailing step/edge 334. In response to primary coil 10 receiving leading step/edge 332 in the absence of LC resonator C or D adjacent primary coil 10, each secondary coil 12, 14 and/or 40 only produces electrical oscillation or ringing 336. This ringing 336 is caused by the capacitive and inductive characteristics of primary coil 10 and each secondary coil 12, 14 and/or 40 in response to excitation of primary coil 10 by leading step/edge 332 of step signal 330.

[0143] If however, LC resonator C or D is included in float 4″, float 4′″ or second member 210 positioned adjacent primary coil 10, in response to exciting primary coil 10 with leading step/edge 332 of step signal 330, each secondary coil 12, 14 and/or 40 produces a time varying signal 338 having a time varying component 340 and a ringing component 342, corresponding to ringing 336, superimposed on time varying component 340. Time varying component 340 of time varying signal 338 is due to the energy coupled into LC resonator C or D by the excitation of primary coil 10 with step signal 330. More specifically, in response to receiving the leading step/edge 332 of step signal 330, primary coil 10 produces a first step electromagnetic field, having a waveform similar to leading step/edge 332 of step signal 330, which is received by each secondary coil 12, 14 and/or 40 and by LC resonator C or D. This first step electromagnetic field couples energy into LC resonator C or D which begins oscillating at its resonant frequency thereby producing a time varying electromagnetic field which is coupled to each secondary coil 12, 14 and/or 40. The dissipation of ringing component 342 is due to the dissipation of energy input into primary coil 10 and coupled directly into each secondary coil 12, 14 and/or 40 by the excitation of primary coil 10 with the leading edge/step 332 of step signal 330.

[0144] In response to receiving the first step electromagnetic field and the time varying electromagnetic field, each secondary coil 12, 14 and/or 40 produces time varying signal 338. The time varying component 340 of time varying signal 338 corresponds to the oscillation of LC resonator C or D and is related to the position of float 4″, float 4′″ or second member 210 along the longitudinal axis T_(A) of primary coil 10. In contrast, ringing component 342 of time varying signal 338 is related to energy input into primary coil 10 and the energy coupled directly into each secondary coil 12, 14 and/or 40 in response to primary coil 10 receiving step signal 330 and, more particularly, the leading step/edge 332 of step signal 330. Hence, ringing component 342 is not related to the position of float 4″, float 4′″ or second member 210 along the longitudinal axis T_(A) of primary coil 10.

[0145] As shown in FIG. 23c, ringing component 342 of time varying signal 338 commences and dissipates temporally adjacent the leading step/edge 332 of step signal 330. In contrast, time varying component 340 of time varying signal 338 commences temporally adjacent leading step/edge 332 of step signal 330 and gradually dissipates, preferably, after one or more oscillation cycles of LC resonator C or D. The difference in duration of ringing component 342 and time varying component 340 is due to differences in the respective energy storage capacity and damping characteristic of primary coil 10, each secondary coil 12, 14 and/or 40 and LC resonator C or D. Specifically, since each primary coil 10 and secondary coil 12, 14 and/or 40 has less capacity to store energy and more damping characteristic than LC resonator C or D, any energy stored in primary coil 10 or stored in each secondary coil 12, 14 and/or 40 dissipates more rapidly than energy coupled into LC resonator C or D.

[0146] To avoid measuring time varying signal 338 in the presence of ringing component 342, control system 16 delays sampling time varying signal 338 until ringing component 342 dissipates. Preferably, control system 16 delays sampling time varying signal 338 until time varying signal 338 crosses a reference voltage 344, e.g., 0 volts, after ringing component 342 dissipates. Thereafter, control system 16 sample for each secondary coil 12, 14 and/or 40 values of time varying signal 338 over time. For example, control system 16 can sample a first peak value 346 of time varying signal 338 and a second peak value 348 of time varying signal 338 for each secondary coil 12, 14 and/or 40. Alternatively, control system 16 acquires for each secondary coil 12, 14 and/or 40 a plurality of values of time varying signal 338 during all or part of one or more half cycles thereof and determines therefrom an average value or an integral value for each half cycle or part thereof. Since no additional energy is coupled into LC resonator C or D during sampling by control system 16, the amplitude of time varying signal 338 gradually decreases due to the dissipation of energy coupled into LC resonator C or D.

[0147] Control system 16 then calculates for each secondary coil 12, 14 and/or 40 a position signal value which is equal to (i) one of the sampled peak values, e.g., peak values 326 or 328; (ii) a sum or difference between two or more adjacent or non-adjacent peak values, e.g., first peak value 326 and second peak value 328 of time varying signal 338; (iii) an average or integral value of all or part of one half cycle of time varying signal 338; or (iv) a sum or difference of the average or integral value of all or part of each of two or more half cycles of time varying signal 338. This position signal value is related to the position of float 4″, float 4′″ or second member 210 along the longitudinal axis T_(A) of primary coil 10.

[0148] After sampling for each secondary coil 12, 14 and/or 40 a first peak value 346 and second peak value 348 or the plurality of values of time varying signal 338 during all or part of one or more half cycles thereof, control system 16 applies trailing step/edge 334 of step signal 330 to primary coil 10. In response to receiving trailing step/edge 334 of step signal 330, primary coil 10 produces a second step electromagnetic field, having a waveform similar to trailing step/edge 334, which is preferably received by each secondary coil 12, 14 and/or 40 and LC resonator C or D before time varying signal 338 dissipates completely. Hence, LC resonator C or D receives the second step electromagnetic field before the time varying electromagnetic field produced in response to leading step/edge 332 dissipates.

[0149] In response to receiving the second step electromagnetic field in the absence of LC resonator C or D adjacent primary coil 10, each secondary coil 12, 14 and/or 40 produces electrical oscillation or ringing 350 having an amplitude opposite the amplitude of ringing 336 produced in response to receiving the first step electromagnetic field. However, when LC resonator C or D is adjacent primary coil 10, the second step electromagnetic field causes a ringing component 352, corresponding to ringing 350, to be superimposed on the time varying signal 338 of each secondary coil 12, 14 and/or 40. In addition, the second step electromagnetic field reinforces the oscillation of LC resonator C or D thereby reinforcing time varying component 340 of time varying signal 338. Since the transition of trailing step/edge 334 is opposite leading step/edge 332, the amplitude of ringing component 352 is opposite the amplitude of ringing component 342.

[0150] Ringing component 352 commences and dissipates temporally adjacent trailing step/edge 334 of step signal 330 while the reinforced time varying component 340 of time varying signal 338 initially increases in amplitude and gradually dissipates one or more oscillation cycles of LC resonator C or D after trailing step/edge 334 of step signal 330.

[0151] Control system 16 delays sampling time varying signal 338 until it crosses reference voltage 344 after ringing component 352 dissipates. Thereafter, control system 16 samples for each secondary coil 12, 14 and/or 40 values of time varying signal 338 over time. For example, control system 16 can sample another first peak value 354 of time varying signal 338 and another second peak value 356 of time varying signal 338. Alternatively, control system 16 acquires for each secondary coil 12, 14 and/or 40 a plurality of values of time varying signal 338 during all of part of one ore more half cycles thereof after ringing component 352 dissipates and determines therefrom an average value or an integral value for each half cycle or part thereof.

[0152] Next, control system 16 calculates for each secondary coil 12,14 and/or 40 a position signal value which is equal to (i) one of the sampled peak values, e.g., peak values 326 or 328; (ii) a sum or difference between two or more adjacent or non-adjacent peak values, e.g., first peak value 326 and second peak value 328 of time varying signal 338; (iii) an average or integral value of all or part of one half cycle of time varying signal 338; or (iv) a sum or difference of the average or integral value of all or part of each of two or more half cycles of time varying signal 338. Utilizing the calculated position signal value for each secondary coil 12, 14 and/or 40, the control system 16 can determine the position of float 4″, float 4′″ or second member 210 along the longitudinal axis T_(A) of primary coil 10 within one cycle of the winding density distribution of each secondary coil 12, 14 and/or 40. Utilizing two or more secondary coils 12, 14 and/or 40 having different winding density distributions and/or shifted winding density distributions enables control system 16 to accurately detect the position of float 4″, float 4′″ or second member 210 along the length of primary coil 10.

[0153] Preferably, control system 16 sequentially applies step signals 330 to primary coil 10. This enables control system 16 to continuously monitor the position of float 4′″, float 4′″ or second member 210 along the length of tube 2.

[0154] Each step signal 330 is shown as a square wave for illustration purposes since square waves are the easiest to generate. However, the shape of step signal 330 is not to be construed as limiting the invention. In this respect, step signal 330 can have any shape that includes a leading edge and a trailing edge, with the square wave of step signal 330 being shown in FIG. 23a as an example of an envelope of all possible pulse shapes. A gaussian signal (not shown) or a bell-shaped signal 368, shown in phantom in FIG. 23d, is preferred, however, since it generates less ringing in primary coil 10 and each secondary coil 12, 14 and/or 40. Bell-shaped signal 368 includes a leading step/slope 370 and a trailing step/slope 372. In response to receiving leading step/slope 370 of bell-shaped signal 368, primary coil 10 produces a first step electromagnetic field, having a waveform similar to leading step/slope 370 of bell-shaped signal 368, which is received by LC resonator C or D. This first step electromagnetic field couples energy into LC resonator C or D and each secondary coil 12, 14 and/or 40 in the same manner as the first step electromagnetic field produced in response to the leading step/edge 332 of step signal 330. Similarly, in response to receiving the trailing step/slope 372 of bell-shaped pulse 368, primary coil 10 produces a second step electromagnetic field, having a waveform similar to trailing step/slope 372 of bell-shaped pulse 3 68, which is received by LC resonator C or D. This second step electromagnetic field couples energy into LC resonator C or D and each secondary coil 12, 14 and/or 40 in the same manner as the second step electromagnetic field produced in response to the trailing step/edge 334 of step signal 330. The form of leading step/slope 370 and the form of trailing step/slope 372 of bell-shaped signal 368 reduce the amplitude of the time varying component 340 and reduce the amplitude and spectral frequency of ringing components 342 and 352 of time varying signal 338. This reduction in amplitude and spectral frequency of ringing components 342 and 352 is due to the lower energy and spectral content of bell-shaped signal 368 versus the energy and spectral content of step signal 330.

[0155] With reference to FIGS. 24a-24 d, and with continuing reference to FIGS. 3-4 and 16 a-16 d, the control system 16 can also apply to primary coil 10 a periodic signal 380 having for each cycle thereof a leading step/edge 382 and a trailing step/edge 384. In response to primary coil 10 receiving leading step/edge 382 in the absence of LC resonator C or D adjacent primary coil 10, each secondary coil 12, 14 and/or 40 only produces electrical oscillation or ringing 386. This ringing 386 is caused by the capacitive and inductive characteristics of primary coil 10 and each secondary coil 12, 14 and/or 40 in response to excitation of primary coil 10 by leading step/edge 382 of each cycle of periodic signal 380. In response to primary coil 10 receiving trailing step/edge 384 in the absence of LC resonator C or D adjacent primary coil 10, each secondary coil 12, 14 and/or 40 only produces electrical oscillation or ringing 388. This ringing 388 is caused by the capacitive and inductive characteristics of primary coil 10 and each secondary coil 12, 14 and/or 40 in response to excitation of primary coil 10 by trailing step/edge 384 of each cycle of periodic signal 380.

[0156] If however, LC resonator C or D is included in float 4′, float 4′″ or second member 210 positioned adjacent primary coil 10, in response to exciting primary coil 10 with periodic signal 380, each secondary coil 12, 14 and/or 40 produces a time varying signal 390 having a time varying component 392, a ringing component 394 corresponding to ringing 386, and a ringing component 396 corresponding to ringing 388. Ringing components 394 and 396 are superimposed on time varying component 392. The time varying component 392 of time varying signal 390 is due to the energy coupled into LC resonator C or D by the excitation of primary coil 10 with periodic signal 380. More specifically, in response to receiving the leading step/edge 382 of each cycle of periodic signal 380, primary coil 10 produces a first step electromagnetic field having a waveform similar to leading step/edge 382 of each cycle of periodic signal 380. Each first step electromagnetic field is received by each secondary coil 12, 14 and/or 40 and by LC resonator C or D. Each first step electromagnetic field couples energy into LC resonator C or D which begins oscillating at its resonant frequency thereby producing a time varying electromagnetic field which is coupled to each secondary coil 12, 14 and/or 40 to produce time varying component 392. The dissipation of ringing component 394 is due to the dissipation of energy input into primary coil 10 and coupled directly into each secondary coil 12, 14 and/or 40 by the excitation of primary coil 10 with the leading step/edge 382 of each cycle of periodic signal 380.

[0157] In response to receiving trailing step/edge 384 of each cycle of periodic signal 380, primary coil 10 produces a second step electromagnetic field, having a waveform similar to trailing step/edge 384. This second step electromagnetic field is received by each secondary coil 12, 14 and/or 40 and LC resonator C or D at a time to reinforce the oscillation of LC resonator C or D due to leading step/edge 382, thereby reinforcing time varying component 392 of time varying signal 390. Preferably, the frequency of periodic signal 380 and the resonant frequency of LC resonator C or D are the same. Thus, leading step/edge 382 and trailing step/edge 384 of each cycle of periodic signal 380 couple energy into LC resonator C or D at appropriate times such that the amplitude of time varying signal 390 increases gradually and, thereafter, remains relatively constant.

[0158] At an appropriate time 398, control system 16 terminates periodic signal 380. To avoid measuring time varying signal 390 in the presence of ringing components 394 or 396, control system 16 delays sampling time varying signal 390 until ringing component 394 and/or 396 dissipate. Preferably, control system 16 delays sampling time varying signal 390 until time varying signal 390 crosses a reference voltage 400, e.g., 0 volts, after ringing components 394 and 396 dissipate. Thereafter, control system 16 samples for each secondary coil 12, 14 and/or 40 values of time varying signal 390 over time. For example, control system 16 can sample a first peak value 402 of time varying signal 390 and a second peak value 404 of time varying signal 390 for each secondary coil 12, 14 and/or 40. Alternatively, control system 16 acquires for each secondary coil 12, 14 and/or 40 a plurality of values of time varying signal 390 during all or part of one or more half cycles after ringing components 394 and 396 dissipates and determines therefrom an average value or an integral value for each half cycle or part thereof. Since no additional energy is coupled into LC resonator C or D during sampling by control system 16, the amplitude of time varying signal 390 gradually decreases due to the dissipation of energy coupled into LC resonator C or D.

[0159] Control system 16 next calculates for each secondary coil 12,14 and/or 40 a position signal value which is equal to (i) one of the sampled peak values, e.g., peak values 326 or 328; (ii) a sum or difference between two or more adjacent or non-adjacent peak values, e.g., first peak value 326 and second peak value 328 of time varying signal 390; (iii) an average or integral value of all or part of one half cycle of time varying signal 390; or (iv) a sum or difference of the average or integral value of all or part of each of two or more half cycles of time varying signal 390 after ringing components 394 and 396 dissipate. This difference value is related to the position of float 4″, float 4′″ or second member 210 along the longitudinal axis T_(A) of primary coil 10.

[0160] Control system 16 can apply periodic signal 380 to primary coil 10 for a predetermined number of cycles or for a predetermined interval when it is desired to determine the position of float 4″, float 4′″ or second member 210 along the longitudinal axis T_(A) of primary coil 10.

[0161] Periodic signal 380 is shown as a square wave for illustration purposes since square waves are the easiest to generate. However, the shape of periodic signal 380 is not to be construed as limiting the invention. In this respect, periodic signal 380 can have any shape that includes a leading edge and a trailing edge, with the square wave of periodic signal 380 being shown in FIG. 24a as an example of an envelope of all possible periodic signals. Preferably, a sine wave 410, shown in phantom in FIG. 24d, is applied to primary coil 10. Sine wave 410 is preferred since it generates less ringing in primary coil 10 and each secondary coil 12, 14 and/or 40. In response to receiving a leading step/slope 412 of each cycle of sine wave 410, primary coil 10 produces a first step electromagnetic field, having a waveform similar to leading step/slope 412 of sine wave 410, which is received by LC resonator C or D. This first step electromagnetic field couples energy into LC resonator C or D and each secondary coil 12, 14 and/or 40 in the same manner as the first step electromagnetic field produced in response to the leading step/edge 382 of each cycle of periodic signal 380. Similarly, in response to receiving the trailing step/slope 414 of each cycle of sine wave 410, primary coil 10 produces a second step electromagnetic field, having a waveform similar to trailing step/slope 414 of each cycle of sine wave 410, which is received by LC resonator C or D. This second step electromagnetic field couples energy into LC resonator C or D and each secondary coil 12, 14 and/or 40 in the same manner as the second step electromagnetic field produced in response to the trailing step/edge 384 of each cycle of periodic signal 380.

[0162] As shown in FIG. 24d, the leading step/slope 412 of each cycle of sine wave 410 extends from the negative peak amplitude of sine wave 410 to the positive peak amplitude of sine wave 410. The trailing step/slope 414 of each cycle of sine wave 410 extends from the positive peak amplitude of sine wave 410 to the negative peak amplitude of sine wave 410. The leading step/slope 412 and the trailing step/slope 414 of each cycle of sine wave 410 reduce the amplitude of the time varying component 392 of time varying signal 390 and reduce the amplitude and spectral frequency of ringing components 394 and 396. This reduction in amplitude and spectral frequency of ringing components 394 and 396 is due to the lower energy and spectral content of sine wave 410 versus the energy and spectral content of periodic signal 380.

[0163] Exciting primary coil 10 with pulse signal 310, half sine wave 329, step signal 330, bell-shaped signal 368, periodic signal 380 or sine wave 410 and sampling values of time varying signal 318, 338 and/or 390 over time, in the manner discussed above in connection with FIGS. 22a-24 d, enables the position of float 4″, float 4′″ or second member 210 along the length of primary coil 10 to be detected while avoiding measurement errors caused by the construction of the displacement measurement system A of the present invention or by the interaction thereof with the measuring environment.

[0164] The invention has been described with reference to the preferred embodiments. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. For example, in the foregoing description, pulse signal 310, half sine wave 329, step signal 330, bell-shaped signal 368, periodic signal 380 or sine wave 410 are utilized to excite primary coil 10. However, this is not to be construed as limiting the present invention which can also utilize signals having other waveforms not specifically described herein. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof. 

I claim:
 1. A sensor system for measuring displacement, the sensor system comprising: a primary coil wound around a longitudinal axis; at least one secondary coil wound around the longitudinal axis, each secondary coil having a winding density distribution that varies between the ends thereof, each secondary coil having a winding direction that varies between a clockwise winding direction and a counterclockwise winding direction between opposite ends thereof; a coupler positioned adjacent the primary coil between the ends thereof, the coupler including a resonating circuit configured to resonate at a resonating frequency; and a control system which excites the primary coil with a first step of a signal and which receives from each secondary coil in response thereto a time varying signal, wherein: at least one of (i) the coupler and (ii) the primary and secondary coils are configured to move relative to the other of the coupler and the primary and secondary coils; each time varying signal includes a ringing component superimposed on a time varying component temporally adjacent the first step of the signal; and the control system acquires one or more values of each time varying signal after the ringing component thereof dissipates and determines therefrom a position of the coupler along the longitudinal axis.
 2. The sensor system as set forth in claim 1 , wherein the control system determines from the one or more values of each time varying signal a peak value thereof and determines therefrom a position signal value related to the position of the coupler along the longitudinal axis.
 3. The sensor system as set forth in claim 1 , wherein the control system: determines of each time varying signal a first peak value after the ringing component dissipates and acquires a second peak value after the first peak value; determines for each time varying signal a position signal value from the first peak value and the second peak value; and determines from the position signal value for each time varying signal a position of the coupler along the longitudinal axis.
 4. The sensor system as set forth in claim 3 , wherein: the first peak value is one of a positive peak value and a negative peak value; and the second peak value is the other of the positive peak value and the negative peak value.
 5. The sensor system as set forth in claim 2 , wherein the at least one secondary coil includes a plurality of secondary coils with their winding density distributions shifted with respect to each other.
 6. The sensor system as set forth in claim 1 , wherein: in response to the first step of the signal, the primary coil produces a first step electromagnetic field; in response to receiving the first step electromagnetic field, the resonating circuit produces a time varying electromagnetic field at the resonating frequency; and in response to receiving the first step electromagnetic field and the time varying electromagnetic field, each secondary coil produces its corresponding time varying signal.
 7. The sensor system as set forth in claim 1 , wherein the resonating circuit includes a resonator coil and a capacitor connected in parallel.
 8. The sensor system as set forth in claim 1 , wherein, after acquiring the one or more values of each time varying signal, the control system excites the primary coil with a second step of the signal having a transition opposite the first step of the signal.
 9. The sensor system as set forth in claim 8 , wherein, after the second step of the signal, the control system: determines from the one or more values of each time varying signal another first peak value after another ringing component thereof dissipates and acquires another second peak value after the other first peak value; determines for each time varying signal another position signal value from the other first peak value and the other second peak value; and determines from the other position signal value for each time varying signal a position of the coupler along the longitudinal axis.
 10. The sensor system as set forth in claim 1 , wherein the signal is a pulse signal or a step signal.
 11. The sensor system as set forth in claim 1 , wherein the control system: determines from the one or more values for all or part of at least one half cycle of each time varying signal an average value or an integral value; determines for each time varying signal a position signal value from the average value or the integral value of the at least one half cycle; and determines from the position signal value for each time varying signal a position of the coupler along the longitudinal axis.
 12. The sensor system as set forth in claim 11 , wherein, after acquiring the one or more values of each time varying signal, the control system: excites the primary coil with a second step of the signal having a transition opposite the first step of the signal; determines one or more other values of each time varying signal after another ringing component thereof dissipates; determines from the one or more other values for all or part of at least one other half cycle of each time varying signal another average value or another integral value; determines for each time varying signal another position signal value from the other average value or the other integral value of the at least one other half cycle; and determines from the position signal value for each time varying signal a position of the coupler along the longitudinal axis.
 13. The sensor system as set forth in claim 1 , wherein in response to the excitation of the primary coil with the first step of the position value signal the coupler produces the time varying component of the time varying signal.
 14. A method of detecting the position of a magnetically susceptible element, the method comprising the steps of: (a) providing a primary coil wound around a longitudinal axis and at least one secondary coil wound around the longitudinal axis at a variable winding density distribution; (b) positioning a coupler along the longitudinal axis adjacent the primary coil and each secondary coil, the coupler including a resonating circuit configured to resonate at a resonating frequency; (c) exciting the primary coil with a first step of a signal; (d) receiving from each secondary coil a time varying signal having a ringing component superimposed on a time varying component temporally adjacent the first step of the signal; (e) acquiring one or more values of each time varying signal after the ringing component thereof dissipates; and (f) determining from the one or more values of each time varying signal a position of the coupler along the longitudinal axis.
 15. The method as set forth in claim 14 , wherein: step (e) further includes the step of determining from the one or more values for each time varying signal a first peak value after the ringing component dissipates and a second peak value after the first peak value; and step (f) further includes the steps of determining for each time varying signal a position signal value from the first peak value and the second peak value and determining from the position signal value for each time varying signal a position of the coupler along the longitudinal axis.
 16. The method as set forth in claim 14 , wherein: step (c) further includes the steps of producing a first step electromagnetic field in response to the primary coil receiving the first step of the signal and producing a time varying electromagnetic field at the resonating frequency in response to the resonating circuit receiving the first step electromagnetic field; and step (d) further includes the step of each secondary coil producing the time varying signal in response to receiving the time varying electromagnetic field.
 17. The method as set forth in claim 14 , further including the steps of: (g) exciting the primary coil with a second step of the signal having a transition opposite the first step of the signal; (h) after the second step of the signal, acquiring other one or more values of the time varying component of each time varying signal after another ringing component thereof dissipates; and (i) determining from the other one or more values of each time varying signal a position of the coupler along the longitudinal axis.
 18. The method as set forth in claim 17 , wherein step (g) further includes the step of exciting the primary coil with the second step of the signal after acquiring the one or more values of the time varying component after the first step of the signal.
 19. The method as set forth in claim 14 , wherein: step (e) further includes the step of determining from the one or more values for each time varying signal a peak value after the ringing component dissipates; and step (f) further includes the steps of determining for each time varying signal a position signal value from the peak value and determining from the position signal value for each time varying signal a position of the coupler along the longitudinal axis.
 20. The sensor system as set forth in claim 1 , wherein: step (e) further includes the step of determining from the one or more values for all or part of at least one half cycle of each time varying signal an average value or an integral value; and step (f) further includes the steps of determining for each time varying signal a position signal value from the average value or the integral value of the at least one half cycle and determining from the position signal value for each time varying signal a position of the coupler along the longitudinal axis.
 21. A displacement measuring sensor system, comprising: a first member extending along a longitudinal axis between a first end and a second end thereof; a primary coil and a secondary coil wound around the longitudinal axis of the first member between the ends thereof; a control system connected for exciting the primary coil with an excitation signal and for receiving a time varying signal from the secondary coil, the time varying signal including a ringing component superimposed on a time varying component temporally adjacent a transition of the excitation signal; a second member positioned adjacent the longitudinal axis and having a resonator which causes the secondary coil to generate the time varying component in response to the excitation of the primary coil with the excitation signal, wherein the control system delays sampling of the time varying signal until the ringing component thereof dissipates.
 22. The system as set forth in claim 21 , wherein one of the first member and the second member are configured to move relative to the other of the first member and the second member.
 23. The system as set forth in claim 21 , wherein: the resonator includes a coil wound around a magnetically susceptible core; the first member is a tube; and the second member is positioned one of inside and outside the tube.
 24. The system as set forth in claim 21 , wherein the second member is a float configured to float on a surface of a liquid. 